As an ignition device that ignites an air-fuel mixture in a combustion chamber of an internal combustion engine, an ignition device is known which ignites by using non-equilibrium plasma (see, e.g., Japanese Patent Application Laid-Open (kokai) No. 2014-123435). Such an ignition device includes an ignition plug having an insulator enclosing a center electrode, and generates non-equilibrium plasma on the surface of the insulator by applying an AC voltage to the center electrode or applying a pulse voltage a plurality of times to the center electrode.